1. Field of the Invention
This invention relates to a brushless motor in which a plurality of magnetically responsive elements such as Hall elements are used to detect the rotating position of a permanent magnet rotor driven by driving coils.
2. Description of the Prior Art
In conventional motors of this kind, a plurality of driving coils are disposed in an equally circumferentially spaced relationship around a permanent magnet rotor securely mounted on a shaft for rotation, and a plurality of magnetically responsive elements, for example, Hall elements are disposed opposite to the outer periphery of the permanent magnet rotor for detecting the rotating position of the rotor.
The driving coils are connected to a d.c. power source through respective control transistors. The Hall elements are also connected to the d.c. power source for the on-off control of the control transistors by the electromotive force induced in the Hall elements.
When the north pole (or south pole) of the permanent magnet rotor is brought to the position opposite to a first Hall element with the rotation of the rotor, a voltage is induced in the first Hall element and the first control transistor connected thereto is turned on to energize the associated driving coil. The permanent magnet rotor is thus urged further in the rotating directon by the driving force imparted by the driving coil until the north pole (or south pole) thereof is brought to the positive opposite to a second Hall element. As a result, a voltage is induced in this second Hall element, and the second control transistor connected to the second Hall element is turned on. At this time, the output voltage of the first Hall element is reduced and the first control transistor is cut off. In this manner, the driving coils are successively energized to cause continuous rotation of the permanent magnet rotor.
In such a brushless motor, there is an important relation between the angle of magnetization of the permanent magnet rotor and the conduction angle which is an angular representation of time, during which current is supplied to the driving coil.
Describing more specifically, the Hall element generates an output voltage whose characteristic is in accord with the density distribution of the magnetic flux produced by the permanent magnet rotor. Therefore, in order that the motor can operate with a high efficiency, the transistor is desirably turned on at the time when the density of the above-mentioned flux is substantially maximum, which means that the transistor is set so as to be turned on only when the output voltage of the Hall element is rather high. However, setting of the operating point for the transistor at such a high voltage value would result in an extremely unstable motor operation in view of the fact that the output voltage of the Hall element tends to be reduced to such an extent that sufficient current cannot be supplied to the driving coil through the transistor due to non-uniformity of the property of the Hall element and permanent magnet rotor and also due to temperature variations. It has therefore been difficult for the prior art motor of this kind to operate satisfactorily efficiently since the conduction angle of the driving coil is equal to or smaller than the angle of magnetization of the permanent magnet rotor.